FIG. 1 shows a conventional vertical light emitting diode (LED), which includes a sandwich structure formed by an N-type semiconductor layer 1A, an active layer 2A and a P-type semiconductor layer 3A. Below the P-type semiconductor layer 3A, a substrate 4A and a P-type electrode 5A are sequentially formed. An N-type electrode 6A is disposed at a surface of the N-type semiconductor layer 1A.
By applying a forward bias between the N-type electrode 6A and the P-type electrode 5A, the N-type semiconductor layer 1A and the P-type semiconductor layer 3A provide electrons and holes, respectively. The electrons and holes are combined in the active layer 2A to perform energy level jump to further generate an excited light having a constant wavelength.
FIG. 2 shows a conventional horizontal LED, which similarly includes a sandwich structure formed by an N-type semiconductor layer 1B, an active layer 2B and a P-type semiconductor layer 3B. The N-type semiconductor layer 1B is formed on a substrate 4A, and a P-type electrode 5B and an N-type electrode 6B are respectively disposed at same sides of the P-type semiconductor layer 3B and the N-type semiconductor layer 1B. By applying a voltage between the N-type electrode 6B and the P-type electrode 5B, electrons and holes are combined in the active layer 2B to generate an excited light.
Referring to FIG. 3, the U.S. Pat. No. 7,223,988, “White, Single or Multi-color Light Emitting Diodes by Recycling Guided Modes”, discloses a photon recycling LED that mainly includes a nitride LED 9. On the nitride LED 9, a non-doped semiconductor layer 9A, a nitride photoluminescent active layer 9B and another non-doped semiconductor layer 9A are sequentially stacked. An electroluminescent light 9C (blue light) generated by the nitride LED 9 enters the nitride photoluminescent active layer 9B to generate a photoluminescent light 9D (yellow) having another wavelength. In other words, the presence of the nitride photoluminescent active layer 9B is to generate a photoluminescent light having another wavelength, i.e., lights of multiple colors are allowed to be blended, and further blended with a red light from a red light emitting substance, a photoluminescent red phosphorescent light emitting substance or red quantum dots, thereby generating a white light with a preferred color rendering index (CRI).
In the prior art, the nitride photoluminescent active layer 9B is provided to absorb the electroluminescent light 9C (blue light) to accordingly generate the photoluminescent light 9D (yellow light), followed by blending the lights to produce a white light having a preferred CRI. However, the actual white light includes colors ranging from red, orange, yellow, green, blue, indigo to violet. It is obvious that, the white light produced by the prior art does not include some of the colors of the actual white light, and lacks a realistic CRI that fully matches the requirements of the actual white light.